Resistance welding is now widely used in most applications requiring the joining of metals, such as the steel used in the manufacturing of automobiles. With the advent of the microprocessor, weld controllers have become more sophisticated and use a variety of control techniques to ensure the quality of welds throughout the life of the contact tips as they wear out. Regardless of the process or control technique used, most weld controllers consist of several basic components. These include a weld control module, a power module, a weld transformer and the contact tips. The power module usually consists of power semiconductors such as silicon controlled rectifiers (SCRs) that switch incoming power to the weld transformer according to a preset weld program as generated by the control module. The weld transformer will transform the incoming power to a high current pulse that is coupled to the contact tips to create a weld to a workpiece that is between the contact tips. The weld control module is usually microprocessor based.
The preset weld program will use phase angle control to switch the power modules. In order to achieve accurate repeatability of the proper phase angle to fire the SCRs, a stable timebase reference is required. This timebase should be immune to the electrical noise generated by other equipment that may be coupled to the power source. The voltage source may have significant line impedance relative to the generally high currents involved with weld controllers. This will cause phase distortion in the incoming voltage that will effect the timebase reference, which, in turn, will cause the SCRs to switch at a different phase angle than the desired angle. Some type of phase distortion compensation is required to ensure that this not occur.
The traditional approach to generating a time base involves measuring the zero crossings of the voltage waveform directly, either by sampling the polarity of the input voltage waveform on a regular basis and determining where in time the input voltage waveform transitions from one polarity to another, or by developing circuitry which determines the zero crossing of the waveform and generating an interrupt to the microprocessor in response to the zero crossings. The electronic circuitry which accomplishes this is commonly referred to as a phase discriminator, and usually involves heavily amplifying the line voltage and clipping the result. The transitions of the resulting waveform are assumed to coincide with the zero crossings of the assumed sinusoid. However, line voltages in automotive welding applications are rarely pure sinusoids, as equipment drawing current from the weld bus can corrupt the line voltage, injecting noise and other distortion which severely limits the reliability of this traditional approach. As an example, weld applications in which weld transformers with full wave rectified secondaries are utilized can cause multiple zero crossings of the observed line voltage in the vicinity of the actual zero crossings of the sinusoid as generated by the power company. Additionally, the presence of line impedance causes the apparent phase of the weld voltage as seen by the weld controller to shift while welding. The impedance of the weld bus, comprising bus fusing, interconnecting wiring and other impedances affect the voltage which appears at the sensing terminals of the weld controller. It is this voltage which is sensed by the weld control electronics and used to derive the phase reference signals which form the basis for firing the solid state weld contactor. In the absence of current flow, it is identical to the bus voltage, but when the weld control switches current to the load, significant harmonic distortion can result which, without detection and appropriate compensation, can result in a degradation of the transient response of the weld control, and can cause the weld control to make errors in estimation of the load power factor. The problem is further complicated in that the weld controller does not apply current continuously, but rather switches current on and off during the cycle. The result is that the amount of phase distortion incurred at the weld control relative to the bus voltage source is dependent not only upon the line and load impedance, but also on the firing angle employed. Zero crossing based phase reference generators significantly limit the performance of the weld control in terms of noise immunity and transient response.
An early type of a phase reference generator which improves upon the traditional phase discrimminator approach above is disclosed in U.S. Pat. No. 4,301,351 which describes an approach for developing a timing reference based on integrating the line voltage to develop a signal proportional to the volt-time area of the waveform. The voltage is sampled four times per nominal period of an internal phase reference. A signal indicative of a change in frequency or phase is generated by taking the difference between the volt-time area of two quarter cycles. In the actual implementation of this patent, all four samples of a line voltage cycle are used to develop an indication of an error in phase between the internal time base and the input line voltage. Errors are estimated based on the previous cycle of line voltage which is then used to compute a correction to the internal phase clock, which is then applied to the next cycle of line voltage. In the specification of the patent, it is indicated that this approach has the advantage of ignoring spurious noise generated on the actual power system by other equipment such as motors and other weld controls. Whereas this integral approach offered significant improvement over the more traditional derivative approach in the presence of a noisy weld bus, as it did not rely on the zero crossing directly to generate the internal phase reference, no attempt was made to quantify the phase error so conventional feedback control techniques available to persons skilled in the art could be employed. Also, the transport lag inherent in this approach due to the number of samples required resulted in the correction occurring in the next cycle, which limited the effectiveness of the approach. Furthermore, the approach described in U.S. Pat. No. 4,301,351 did not provide for any estimate of, or compensation for, the phase distortion inflicted upon the observed line voltage due to the weld process, which is of a more repetitive nature. It would be desirable to develop a system or method whereby this time lag is reduced and the effect of phase distortion can be compensated, resulting in an internal phase reference which more accurately tracks the actual voltage source, rather than the distorted voltage signal observed by the weld control.